Traditionally, in a power station installation, a low-pressure turbine is supplied by a duct with steam at a pressure of 3 to 6 bar and at a temperature of the order of 150° C. and the steam needs to be as dry as possible, and it emits this steam on the outlet side at a pressure of 40 to 150 millibar and a temperature of the order of 25° C. What is meant by “dry steam” is that the steam is in gaseous form and contains no (or practically no) droplets in liquid form. In theory, a flow of steam entering the LP turbine contains no moisture (or in other words needs to be dry, to contain no steam in the liquid state), whereas a flow of steam leaving the turbine contains between 8 and 16% moisture.
The design of such a turbine means that the steam inlet duct is partially comprised within the casing of the turbine and thus immersed within the turbine in the flow of exiting steam.
The result of this is that there are thermal interactions between the steam inlet duct and the flow of steam leaving the turbine, and these cause the steam arriving at the turbine to be cooled, thus increasing its moisture content.
Now, in order to have the best possible efficiency, it is necessary to have the driest possible steam in the supply duct, which preferably means steam with a moisture content of 0%.
It is therefore necessary to limit the thermal interactions between the steam inlet duct and the flow of steam leaving the turbine.